Location

Location ANSS

The ANSS event ID is usp000b5kb and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000b5kb/executive.

2002/06/05 20:17:36 52.890 -74.410 18.0 4.5 Quebec, Canada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2002/06/05 20:17:36:0 52.89 -74.41 18.0 4.5 Quebec, Canada Stations used: CN.A11 CN.A21 CN.A64 CN.ICQ CN.KAPO CN.SCHQ CN.VLDQ Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 4.17e+21 dyne-cm Mw = 3.68 Z = 3 km Plane Strike Dip Rake NP1 149 47 75 NP2 350 45 105 Principal Axes: Axis Value Plunge Azimuth T 4.17e+21 79 345 N 0.00e+00 11 159 P -4.17e+21 1 249 Moment Tensor: (dyne-cm) Component Value Mxx -3.82e+20 Mxy -1.41e+21 Mxz 7.51e+20 Myy -3.64e+21 Myz -1.32e+20 Mzz 4.03e+21 ######-------- #############--------- --################---------- ---##################--------- ----####################---------- -----#####################---------- ------######################---------- -------#######################---------- -------########### #########---------- --------########### T ##########---------- ---------########## ##########---------- ----------######################---------- ----------######################---------- --------#####################--------- P ---------####################--------- -----------##################-------- ------------################-------- -------------##############------- -------------###########------ ---------------#######------ ---------------------- ------------## Global CMT Convention Moment Tensor: R T P 4.03e+21 7.51e+20 1.32e+20 7.51e+20 -3.82e+20 1.41e+21 1.32e+20 1.41e+21 -3.64e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020605201736/index.html

Preferred Solution

      STK = 350
      DIP = 45
     RAKE = 105
       MW = 3.68
       HS = 3.0

The NDK file is 20020605201736.ndk The waveform inversion is preferred.

Context

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.

Location of broadband stations used for waveform inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   275    70    -5   3.54 0.5130
WVFGRD96    1.0   280    65    10   3.56 0.5325
WVFGRD96    2.0   280    70    20   3.61 0.5653
WVFGRD96    3.0   350    45   105   3.68 0.5685
WVFGRD96    4.0   100    70    25   3.66 0.5396
WVFGRD96    5.0   270    60   -25   3.67 0.5299
WVFGRD96    6.0   270    60   -20   3.67 0.5374
WVFGRD96    7.0   270    60   -20   3.67 0.5411
WVFGRD96    8.0   270    60   -20   3.67 0.5413
WVFGRD96    9.0   270    60   -20   3.67 0.5390
WVFGRD96   10.0   270    60   -20   3.69 0.5365
WVFGRD96   11.0   270    60   -20   3.69 0.5303
WVFGRD96   12.0   270    60   -25   3.70 0.5319
WVFGRD96   13.0   270    60   -25   3.70 0.5335
WVFGRD96   14.0   270    60   -25   3.71 0.5334
WVFGRD96   15.0   270    60   -25   3.72 0.5316
WVFGRD96   16.0   270    60   -25   3.72 0.5293
WVFGRD96   17.0   270    60   -25   3.73 0.5266
WVFGRD96   18.0   280    60   -35   3.71 0.5243
WVFGRD96   19.0   280    60   -35   3.72 0.5220
WVFGRD96   20.0   270    55   -30   3.76 0.5144
WVFGRD96   21.0   280    55   -40   3.74 0.5101
WVFGRD96   22.0   280    55   -40   3.75 0.5050
WVFGRD96   23.0   280    60   -45   3.75 0.4998
WVFGRD96   24.0   280    60   -45   3.76 0.4935
WVFGRD96   25.0   280    55   -40   3.77 0.4866
WVFGRD96   26.0   280    55   -40   3.78 0.4793
WVFGRD96   27.0   270    50   -35   3.81 0.4716
WVFGRD96   28.0   270    50   -35   3.81 0.4639
WVFGRD96   29.0   270    50   -35   3.82 0.4561

The best solution is

WVFGRD96    3.0   350    45   105   3.68 0.5685

The mechanism corresponding to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 

Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Velocity Model

The CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

MODEL.01
CUS Model with Q from simple gamma values
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
  H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC)   QP   QS  ETAP  ETAS  FREFP  FREFS
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00